The design of high-accuracy analog circuits is becoming a difficult task with the scaling down of supply voltages and transistor channel lengths of the current mixed-signal integrated circuits. Different circuits require the use of a high performance active cell as the operational amplifier (OpAmp). Furthermore, some low-IF transceivers use a complex filter requiring an operational amplifier with a very high gain-bandwidth product in order to have enough loop gain at the higher band limit and low power circuits impose a reduction in the current in order to increase the battery life-time in mobile handsets.
Unfortunately, the bandwidth depends on the technology and the current used in the OpAmp output stage and hence the only way to reduce the current consumption is to reduce the complexity/consumption of the circuits around the OpAmp such as bias, common mode feed-back, start-up circuit.
A good solution to the above requirements is represented by the fully-differential topology OpAmp; in fact the fully-differential topology guarantees a high level of common-mode noise and spur-harmonics rejection. However, these operational amplifier topologies still require a common-mode circuit which represents an additional source of current consumption; in some cases an undesired bias condition can be verified for reducing this current consumption.